If you’re having a hard time finding a character’s voice, get him talking about something unrelated to the scene at hand.
Let your hero knock back a beer with his college roommate. Have your corporate spy meet-cute a potential suitor at a ski lodge. Pick situations that couldn’t possibly fit in your actual movie. You just want to get your character talking so that you can eavesdrop.
Open a new document and start typing.
It can be a monologue or a discussion between several characters, but go for pure dialogue, no scene description. Let it ramble on for one page or twenty. Again: you’ll never use this, so there’s no pressure to get it right or tight.
Just like a painter will often do sketches and studies before embarking on a major portrait, writing “off the page” lets you figure out what’s interesting about your character before you burden her with plot. It’s also fun. It’s the easy part of screenwriting you imagined before you actually sat down to do it.
